feat(sentry-core): add support for w3c traceparent

[saiintbrisson]

This change does three things:

1. Add a function to allow creating a TransactionContext with a custom Trace ID without a parent span ID.
   My company is testing traces on Sentry in parallel to our current trace solution, for ease of use, we'd like to keep using the same trace ID on both platforms for now. Currently, my approach is to create a sentry-trace header with a zeroed span ID, but the UI has this bright red alert telling me that the parent span ID is invalid, and this is bugging me. I doubt I'm the only one who has faced this issue before.
2. Implements support for the traceparent as stated in Tracing: W3C Trace Context Support team-sdks#41. sentry-trace still takes precedence. It does not emit the header, though. Should it? It only allows for parsing for now.
3. Creates a builder allowing to customize the creation of a transaction.

[saiintbrisson]

I need to figure some stuff out in #686 before doing this one.

[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 4 lines in your changes missing coverage. Please review.

Project coverage is 73.26%. Comparing base (e3b0fba) to head (4ca67ff).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #687      +/-   ##
==========================================
+ Coverage   73.25%   73.26%   +0.01%     
==========================================
  Files          66       66              
  Lines        7754     7781      +27     
==========================================
+ Hits         5680     5701      +21     
- Misses       2074     2080       +6     

[saiintbrisson]

@Swatinem this one is also ready to be reviewed.